High pressure pulsed gas sources derived by electrothermal techniques are disclosed, for example, in commonly assigned U.S. Pat. Nos. 4,590,842, 4,715,261, 4,974,487 and 5,012,719. Some of these prior art devices avoid the need for energetic chemicals that frequently become unstable and pose a constant safety problem. In these prior art pulsed gas sources, a capillary discharge is formed in a passage between a pair of spaced electrodes at opposite ends of a dielectric tube, preferably formed of polyethylene. In response to a discharge voltage between the electrodes, a high pressure, high temperature plasma fills the passage, causing material to be ablated from the dielectric wall. High temperature, high pressure plasma gas flows longitudinally of the discharge and the passage through an aperture defined by an electrode at one end of the passage. The gas flowing longitudinally from the passage through the aperture produces a high pressure, high velocity gas jet that can accelerate a projectile to a high velocity. In the '487 patent, the high pressure, high temperature plasma interacts with a propellant mass to produce a high temperature propellant. In the '719 patent, hydrogen is produced by interacting the plasma flowing through the orifice with a metal hydride and some other material to produce high pressure hydrogen. The plasma is cooled by interacting with a cooling agent, for example water, while an exothermal chemical reaction is occurring.
In the '487 patent, the pressure acting on the rear of a projectile is maintained substantially constant while the projectile is accelerated through a barrel bore even though the volume of the barrel bore between the high pressure source outlet orifice and the projectile increases. Such a result is attained by increasing the electric power applied to the capillary discharge in a substantially linear manner as a function of time.
In still a further high pressure pulsed gas source disclosed in commonly assigned U.S. Pat. No. 5,072,647, a high pressure plasma discharge is established between a pair of axially displaced electrodes. The pressure of the plasma in the discharge is sufficient to accelerate a projectile in a gun barrel bore. The plasma is established in a walled structure confining the discharge and having openings through which the plasma flows transversely of the discharge. A chamber surrounding the wall includes a slurry of water and metal particles to produce high pressure hydrogen gas that flows longitudinally of the discharge against the rear of a projectile. To maintain the pressure of the hydrogen gas acting against the projectile relatively constant as the projectile is accelerated down the barrel, electric power applied to the discharge increases substantially linearly as a function of time.
Some concepts employed in the '647 patent have been incorporated into the co-pending, commonly assigned application Ser. No. 08/238,433, filed May 5, 1994. In this co-pending application, a structure establishes at least several axial electrical discharges across axial gaps behind an outlet of a high pressure pulsed gas source, particularly adapted for driving a projectile. The discharges cause plasma to flow with components at right angles to the axial discharges. A conventional propellant mass, e.g. gunpowder, or a hydrogen producing mass, as disclosed in the '647 patent, is positioned to be responsive to the plasma flow resulting from the discharges. In response to the plasma resulting from the discharges being incident on the propellant mass, a high pressure gas pulse is produced.
Those working in the art have recognized that it is desirable for plasma accelerating a projectile to have a maximum amount of pressure close to the base, i.e., rear, of the projectile. Hence, after a projectile is initially accelerated, it is desirable for the power close to the projectile, at the front of a plasma source, to be greater than the power at the rear of the plasma source. However, a problem in producing a plasma with such a power or energy distribution is that pressure waves have a tendency to be produced in the plasma source. The pressure waves from a high pressure plasma source, such as derived from a highly energetic electric power supply (having millions of Joules of energy), can be destructive of a projectile launcher including such a high pressure source. It is, therefore, desirable for a high pressure plasma source having at least several axial electrical discharges to initially produce plasma having about the same power over all of the gaps. After the projectile has moved away from its initial position, it is then desirable for the power applied to the plasma close to the projectile to exceed the power of the plasma farther from the projectile.
A problem with the aforementioned types of devices is that the plasma has a tendency to flow through a plasma confining structure to an electrode needed to establish the axial electrical discharges; the electrode must be at a high voltage relative to metal parts close to it. If the plasma has a high temperature at the time it is incident on the electrode, many charge carriers are incident on the electrode, causing a low impedance electric path to subsist between the electrode and the metal parts. This constitutes a parallel current path to the desired discharges, diverting current away from the desired discharges. The original electric discharges thus have a tendency to be quenched. To overcome this problem in the past, it has been the general practice to design the structure so the electrode is a great distance from the discharge structure. Such an arrangement enables the high temperature of the plasma to be largely dissipated to reduce the number of plasma charge carriers incident on the electrode. However, such a lengthy structure is not conducive to optimum design of cartridges including projectiles adapted to be loaded into military hardware.
Many of these problems are considered and solved in the co-pending, commonly assigned application of Goldstein et al. (08/329,755 entitled "HYBRID ELECTROTHERMAL GUN WITH SOFT MATERIAL FOR INHIBITING UNWANTED PLASMA FLOW AND GAPS FOR ESTABLISHING TRANSVERSE PLASMA DISCHARGE," filed Oct. 26, 1994. In that application, there is disclosed a high pressure pulsed gas source, particularly adapted to accelerate a projectile along a gun barrel. The source comprises a structure for establishing at least several axial electrical discharges in corresponding axial gaps behind an outlet; the projectile is initially located immediately in front of the outlet. The discharges cause plasma to flow with components at right angles to the axial discharges for a substantial time while the pulse is being derived and while the projectile is traversing the barrel. A propellant mass positioned to be responsive to the plasma flow resulting from the discharges is converted into a high pressure component of the gas pulse by the plasma. The axial gaps are arranged so that after the pulse is initially formed and is still being derived, i.e., after the projectile moves away from its initial position and is in the barrel, the power applied to the plasma via gaps close to the outlet is greater than the power applied to the plasma via the gaps farther from the outlet. Thereby, greater power and pressure are applied to the base of the projectile, to accelerate the projectile more efficiently and to higher velocities.
To avoid damage or destruction to a structure for deriving the high pressure gas pulse, e.g., the gun including the barrel, the axial gaps are arranged so that when the plasma is initially produced and the pulse is initially derived, the power applied to the plasma is substantially the same in the several discharges.
Preferably, the gaps include walls that erode differently in response to the discharges so that during the application of power to the gaps, the walls of the gaps close to the outlet and projectile erode faster than the walls of the gaps farther from the outlet and projectile. Initially, the power developed in all of the gaps is approximately the same. After a particular discharge structure has been used once, it is discarded, as is common for the accelerating portions of projectile cartridges.
In one embodiment, the walls of the gaps close to the projectile have a smaller radius than walls of the gaps farther from the projectile, to cause greater erosion of the gaps close to the projectile than the walls of the gaps farther from the projectile. A similar result is achieved by arranging the walls of the gaps close to the projectile to be axially closer to each other than the walls of the gaps farther from the projectile. Greater uniformity of the initial application of power to the gaps is provided by combining the two aforementioned factors, i.e., by arranging the walls of the gaps close to the projectile to have a smaller radius than the walls of the gaps farther from the projectile and arranging the walls of the gaps close to the projectile to be closer to each other than the walls of the gaps farther from the projectile. The gap length and wall radii can be changed gradually from gap to gap or half of the gaps close to the outlet can have the same first configuration while the gaps remote from the outlet can have a second configuration which differs from that of the first configuration.
The use of gaps having different geometries is predicated on the proposition that during a discharge, all of the gap lengths increase. The increase in length of the smaller gaps is greater than the increase in length of the larger gaps. Thus, there is a shift in the plasma power toward the front of the plasma source where the small gaps are located. In a similar manner, as the radial length of the gap walls increases, the resistance of the plasma in the gap decreases, resulting in lower power dissipation in the gap for equal length gaps. The decreased power dissipation in the gaps having the thicker walls results in less erosion from these walls and thereby causes less erosion of the thicker walls farther from the projectile than the thinner walls closer to the projectile.
Preferably, each wall is part of a member having an outer periphery beyond the wall. The outer periphery is formed of a non-electrically conducting material that is eroded by the plasma at a rate which is slower than the wall material. The outer periphery thus retains its geometry during the discharge so plasma incident on the outer surface thereof does not change the discharge structure. This feature also provides predictable plasma flow characteristics from the discharge structure into the propellant.
The electrical power supply connected to the structure enables the pressure applied to the projectile to remain approximately constant while the projectile is being accelerated in the barrel, even though the volume in the barrel between the outlet of the high pressure source and the base of the projectile is increasing. To these ends, the power supply initially produces a high power electric pulse to initially apply a high pressure plasma from the several discharges to the projectile. Then, after the projectile has moved away from its initial position, a smaller amount of electric power is applied to the gaps. At this time, the stored potential energy in the propellant mass is converted into pressure that is applied to the projectile via the barrel. Electric power applied to the gaps is then ramped upwardly to increase the plasma pressure and the pressure resulting from the converted propellant mass, such that the total pressure applied to the projectile remains approximately constant from a moment shortly after the discharge is initially generated to the end of the discharge, typically about 1,000 microseconds after the initial discharge occurs.
In this prior art structure, the propellant mass is described as gunpowder. Hence, the safety advantages of the earliest electrothermal devices are not included in the structure of the co-pending application. In addition, the prior art use of gunpowder is not particularly efficient because a fraction of the gunpowder burns too late to affect pressure on the projectile base. Also, the electrical energy may be supplied too late into the pulse, that is during a later portion of the pressure pulse when the pressure gradually drops toward zero.
It is, accordingly, an object of the present invention to provide a new and improved apparatus for and method of deriving a high pressure gas pulse, particularly adapted to drive a projectile in a gun barrel.
Another object of the invention is to provide a new and improved cartridge including a projectile and an electrothermal structure for driving the projectile to high speeds in a gun barrel.
A further object of the invention is to provide a new and improved method of deriving a high pressure gas pulse from a mass of non-gaseous material that is relatively inert, hence safe, at ambient conditions, and which is vaporized and chemically reacted such that a relatively large percentage of the potential energy thereof is converted to kinetic energy.
An additional object of the invention is to provide a new and improved electrothermal apparatus including at least several axially displaced gaps for deriving a plasma that flows radially with respect to a structure including the axial gaps wherein the apparatus employs a propellant mass that is relatively inert at ambient conditions and a very high percentage thereof is converted to kinetic energy.
A further object of the invention is to provide a new and improved cartridge including a projectile attached to a structure for deriving at least several axially spaced plasma jets that flow into a propellant mass that is radially displaced from the structure and is relatively inert at ambient conditions, but has a high percentage of its potential energy converted to kinetic energy.